Application of liquid crystal thermography in heat transfer characteristics of slot jet impingement.

Chan, T.L., 1998. Application of liquid crystal thermography in heat transfer characteristics of slot jet impingement. PhD, Nottingham Trent University.

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A fully automatic non-intrusive liquid crystal thermographic system, based on colour image processing and a transient wall heating technique, has been developed to obtain the surface heat transfer characteristics of a heated slot jet impinging on a semi-cylindrical convex surface. A novel calibration procedure for thermochromic liquid crystals has been developed which allows the technique to be applied with confidence to oblique surfaces for the first time. The parametric effects of jet Reynolds number (Re,w) ranging from 5,600 to 13,200 and the slot jet to impingement surface spacing (Y/W) ranges from 2 to 10 have been studied. Velocity and turbulence measurements of the slot jet exit were determined using hot wire anemometry.

High momentum directly below the jet leads to maximum heat transfer levels at the stagnation point. The flow rapidly decelerates and they begins to accelerate as it is deflected through 90 degrees and a wall jet begins to form. Local Nusselt numbers Nus decrease with increasing circumferential distance S/W from its maximum value at the stagnation point up to S/W= 3.1. The transition in the wall jet from laminar to turbulent flow was completed by about 3.3≤ S/W ≤4.2 which coincided with a secondary peak in heat transfer. The stagnation Nusselt number NU0 shows good agreement with Nu0∝Rew0.5 for laminar boundary-layer flow for Y/W= 2 to 6 in this study. For the larger spacings Y/W=8, which are beyond the potential core, the jet Reynolds number dependence is stronger (Nu0∝Rew0.54), and air entrainment effects on the jet momentum take place in this region. The local circumferential Nusselt numbers increase with increasing Reynolds numbers and the NU0 was shown to increase non-linearly to its peak values at the end of the potential core, Y/W= 8.

The rate of decay of average circumferential Nusselt numbers along the cylindrical convex surface is much faster than that which occurs laterally along the flat surface. New correlations of local and average Nusselt numbers with Rew, Y/W and S/W have been established for the stagnation point and the circumferential distribution. As Y/W increases, the effect of curvature becomes apparent and the difference between the flat surface correlation and the convex surface becomes more pronounced. These provide previously unavailable guidelines for industrialists exploiting jet impingement for optimum heating/cooling applications.

The suitability of the widely used k-e turbulence model, within the commercial code PHOENICS, has been assessed for predicting this flow. In the stagnation region, NU0 was overpredicted by up to 190%. Performance of the model improved in the wall jet region and with careful selection of the near wall cell size, which is crucial to ensure correct implementation of wall functions, Nus was predicted to within 29%.

Item Type: Thesis
Description: This research programme was carried out in the Department of Mechanical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong in collaboration with the Department of Mechanical Engineering, The Nottingham Trent University, Nottingham
Creators: Chan, T.L.
Date: 1998
ISBN: 9781369314182
Rights: © Copyright notice This copy of the thesis has been supplied for the purpose of research or private study under the condition that anyone who consults it is understood to recognise that its copyright rests with its author and that no quotation from the thesis, no information derived from it, may be published without proper acknowledgement.
Divisions: Schools > School of Architecture, Design and the Built Environment
Record created by: Jeremy Silvester
Date Added: 25 Sep 2020 12:42
Last Modified: 20 Jul 2023 11:00

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